Efficient Grid in Qt

Question

I am trying to draw a 10 millisecond grid in a QGraphicsScene in Qt. I am not very familiar with Qt... it's the first time I used it, and only because the application needs to be portable between Windows and Linux.

I actually don't have a problem drawing the grid, it's just the performance when the grid gets big. The grid has to be able to change size to fit the SceneRect if/when new data is loaded into the program to be displayed.

This is how I do it at the moment, I hate this but it's the only way I can think of doing it...

void Plotter::drawGrid() {
    unsigned int i;

    QGraphicsLineItem *line;
    QGraphicsTextItem *text;

    char num[11];
    QString label;

    unsigned int width = scene->sceneRect().width();
    unsigned int height = scene->sceneRect().height();

    removeGrid();

    for (i = 150; i < width; i+= 10) {
        line = new QGraphicsLineItem(i, 0, i, scene->sceneRect().height(), 0, scene);
        line->setPen(QPen(QColor(0xdd,0xdd,0xdd)));
        line->setZValue(0);

        _itoa_s(i - 150, num, 10);
        label = num;
        label += " ms";

        text = new QGraphicsTextItem(label, 0, scene);
        text->setDefaultTextColor(Qt::white);
        text->setX(i);
        text->setY(height - 10);
        text->setZValue(2);
        text->setScale(0.2);

        //pointers to items stored in list for removal later.
        gridList.append(line);
        gridList.append(text);
    }
    for (i = 0; i < height; i+= 10) {
        line = new QGraphicsLineItem(150, i, width, i, 0, scene);
        line->setPen(QPen(QColor(0xdd,0xdd,0xdd)));
        line->setZValue(0);
        gridList.append(line);
    }
}

When scene->sceneRect().width() gets too big, however, the application becomes very sluggish. I have tried using a QGLWidget, but the improvements in speed are marginal at best.

A similar question has been asked before: http://stackoverflow.com/questions/12477082/render-qgraphicsscene-according-to-zoom-level — cmannett85, Sep 24 '12 at 17:21
Tip: Instead of the part with _itoa_s, say `label += QString("%1 ms").arg(i - 150);` — Stefan Majewsky, Sep 25 '12 at 11:43

score 1 · Accepted Answer · answered Sep 25 '12 at 11:37

1

I ended up using a 10x10 square pixmap and drew it as the backgroundBrush on my QGraphicsView, as is suggested in the link in the comment under my initial question.

answered Sep 25 '12 at 11:37

Luke

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Efficient Grid in Qt

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